Construction of joints and sealing ends for high tension electric cables



Sept. 17, 1963 PALMIERl 3,104,275

CONSTRUCTION OF JOINTS AND SEALING ENDS FOR HIGH TENSION ELECTRIC CABLES Filed Aug. 9; 1960 INVENTOR.

vm h ATTORNEYS United States Patent CONSTRUCTION OF .iOINTS AND SEALING ENDS FOR HIGH TENSION ELECTRIC CABLES Nicola Palmieri, Milan, Italy, assiguor to Pirelli Societa per Azioni, Milan, Italy, a corporation of Italy Filed Aug. 9, 1960, Ser. No. 48,529 Claims priority, application Italy Aug. 26, W59 4 Claims. (Cl. 174-73) This invention relates to improvements in the construction of joints and sealing ends for high tension cables and particularly to the means for providing electrical screening in such joints and ends.

When high tension cable ends are joined or terminated, the uniform construction of the cable itself, capable of withstanding high voltages, must be interrupted for joining or terminating purposes, and the final joint or termination must be able to withstand high electrical stresses without breakdown. Any irregularities or sharp discontinuities in the joint or sealing end increase the electrical stresses to which the parts are subjected and should be avoided. Generally, the joints or ends are formed by hand and paper or other insulating tape and nonstretchable semi-conductive tape are employed respectively to rebuild the insulation and to provide electrical screening or stress reduction in the joints or ends.

It is known in the art that the conductor of high tension electric cables, in which the insulation is made of impregnated paper may be wound with tapes of semi-conductive paper having suitable characteristics. It is known also that said semi-conductive paper, besides acting as an electrostatic screen of the conductor, increases the ionization inception or starting stress of the dielectric formed by paper and oil in contact with it. Very often, for analogous reasons, the semi conductive paper is applied also at the outside of the cable insulation. Although the invention has other applications, it is particularly useful in joints or ends for cables of this type.

In the present practice with such cables, both the semiconductive screen on the conductor and the one around the cable insulation are interrupted in the zone of the joints, and it is practically impossible to apply in these zones a good taping of semi-conductive paper tape during the rebuilding of the insulation because of the irregular and/or sharply tapered surfaces which are present for various reasons. Thus, the semi-conductive tape which is inelastic or nonextensible and cannot stretch and conform to the inregul-ar surfaces does not provide a smooth screen and makes it diflicult to' avoid pockets. In fact, the jointing clamp, causing a sudden variation in the. diameter, makes it impossible to properly fit the tape of semi-conductive paper on the metallic surface in the jointing zone. Moreover, it must be added that in the joints of the oil-filled type of cables, in order to avoid an excessive oil leakage during the assembling operations, a plugging of varnished silk is fitted over the conductor at the two ends of the jointing clamp. This material has considerably lower dielectric characteristics as compared to the impregnated paper and is situated in a zone subjected to high electric stresses. Therefore, it can be seen that the working conditions in these zones are critical from the standpoint of the dielectric strength and that the electric screening of said zones is extremely useful.

Likewise, in the joints and sealing ends, the screening of semi-conductive paper outside the insulation, along the surface of the tapered ends of the insulating sleeve of the joint, cannot be rebuilt owing to the impossibility of applying a usual paper screening tape on the noncylindrical surfaces of said tapered ends. In this case the screening is carried out in prior practice with a wire helically wound up in close coils, which is directly applied ice 2 on the insulating paper, or with a braid of thin copper wires, so as to form a stress distributing cone.

According to the present invention, the above mentioned difiiculties are overcome by the use of tapes of semi-conductive extensible or stretchable paper which may be stretched or extended an amount considerably in excess of the amount which the previously used semi-conductive tapes could be stretched. Such paper is known in the art and may be for example, paper known as crepe paper which is marked, creased or otherwise treated so as to give it good elastic and stretch properties. The elongation of the paper, without breaking, should be at least 20 percent to provide forming properties which will permit the tape to conform to irregularities normally encountered and may be as high as percent. Preferably, the elongation is in the range from about 50 to 70 percent.

Preferably, the paper employed for making the semiconductive stretchable tape is made by the well known processes employed for making semi-conductive paper, and it is then creped in the manner commonly employed for making crepe paper. Thus, the known types of semiconductive tapes, used for screening in high tension cables, or the paper from which it is made, may be creped or otherwise processed in a conventional manner to produce a semi-conductive stretchable, elastic tape having an elongation of at least 20 percent which is far in excess of the elongation of the uncreped tape or paper. Furthermore, such treatment of the semi-conductive tape or paper has been found to have no adverse effect on the conducting and screening properties of the resulting tape. As used herein and in the appended claims, the term stretch paper and semi-conductive stretchable paper tape mean respectively paper which has stretch properties several times greater than the stretch properties of paper previously used to make semi-conductive paper tape and to semi-conductive tape made from such stretch paper. Similarly, the term semi-conductive creped paper tape means tape made from semi-conductive paper which has been creped to provide stretch properties several times greater than the stretch properties of paper previously used to make semi-conductive paper tape.

The attached drawing illustrates by way of non-limiting example a joint for high tension cables carried out in accordance with the invention, Sealing ends are similarly constructed except that one cable is replaced by another conductor or is omitted and the end may correspond to one-half of a joint. In said drawing:

FIG. 1 represents a joint in axial section;

FIG. 1a represents in enlarged detailed view the entrance of the conductor of one of the two lengths to be joined into the jointing clamp, in the case of a solid type cable;

and

FIG. 1b represents in enlarged detailed view the entrance of the conductor of one of the two lengths to be joined into the jointing clamp, in the case of an oil-filled cable.

In the jointing zones of the cable, the semi-conductive stretchable paper tape is applied on the conductor 1 so as to ensure again the continuity of the screening on the clamp 2. The taping 3, formed by the tape of semiconductive stretch paper, starts from the zone where the semi-conductive paper 4 (which may be conventional non-stretchable paper) of one of the two cable lengths to be joined has been cut, and terminates in the corresponding zone of the other length, after having covered the whole clamp 2, care being taken that the semi-conductive stretch paper 3 be electrically connected with the semiconduotive paper screen 4.

In order to facilitate the application of the tape in the transition zones between the clamp and each conductor, these zones can be wrapped with a filling 5 (FIG.

1a) made of the same semi-conductive stretch paper, 7

so as to obtain a gradual variation of the diameter.

When the two sides of the clamp are wrapped with a plugging 6 of varnished silk (FIG. lb), the semi-conductive stretchable paper can be directly applied over said plugging. In this Way the silk is removed from the action of the electric field.

Before winding the Wire '8, made of metal or of any other conductive material, on the tapered end of the insulating sleeve 7 of the joints (FIG. 1) or of sealing ends, a suitable winding 9 can be carried out with the tape of semi-conductive stretch paper, star-ting from the zone where the semi-conductive outer screen 10* of the cable has beeninterrupted, so as to maintain the continuity of the screening. In this Way the danger deriving from the imperfections and irregularities of the winding of metal wire, which forms the stress cone will be avoided and, when a braid of copper wires is usedto this end, the

possible irregularities and metallic points will be screened. It will be apparent from the foregoing that in the constructions shown, the semi-conductive stretchable paper tape may be readily applied in a relatively thin layer and not only will conform readily to both the cylindrical and non-cylindrical surfaces but also will substantially eliminate discontinuities in the screening effect and present a substantially smooth surface so as to mini mize electrical stress concentrations.

What I claim is: l. A high tension cable'structure at an end of a cable having a central conductor, said structure'comprising at least one conductive part electrically connected to and encircling said conductor, said part having an exterior surface spaced from said-conductor and extending substantially parallel to the surface of said conductor and having an end surface extending at an angle to and between said exterior surface of said part and said conductor surface, a'filling around said conductor and abutting said end surface at one end thereof, the exterior surface of said filling, at said one end thereof, being substantially at the level of said exterior surface of said part and, at the other end thereof, being substantially at the level of said conductor surface, said exterior surface of said filling having a gradual taper intermediate said ends thereof, and a winding of semi-conductive stretch paper around said filling and having an exterior surface extending from said exterior surface of said part to the surface of said conductor.

2. A high tension cable joint'inter'connecting a pair of cables each having a central conductor wound with semiconductive paper, said joint comprising a jointing clamp extending over and joining the end of the conductor of one cable with the end of the conductor of the other cable, said clamp having an intermediate exterior surface spaced from the conductors of said cables and extending substantially parallel to the surfaces of said conductors, and having end surfaces extending at an angle to and between said exterior surface of said clamp and said conductor at opposite ends of said clamp, each filling abutting an end surface of said clamp at oneend thereof, the exterior surface of said filling, at said one end thereof, being substantially at the level of said exterior surface of said clamp and, at the other end thereof, being substantially at the level of the conductor surface, the exterior surface of each said filling having a gradual taper intermediate said ends thereof, and a 'winding of semi-conductive stretch paper around said clamp and said fillings and extending over the ends thereof and into contact with said first mentioned semi-conductive paper. 3. A high tension cable joint interconnecting a pair of cables each having a central conductor, said joint comprising a jointing clamp having an interior bore receiving the ends of the conductors of said cables, said clamp having an intermediate exterior surface of greater circumference than the circumference of said conductors spaced from the surfaces of said conductors and extending substantially parallel to the surfaces of said conductors and having end surfaces extending at an angle to and between said exterior surface of said clamp and said surfaces of said conductors, a filling of semi-conductive stretch paper around each of said conductors and abutting an end surface of said clamp, each filling having a tapered exterior surface extending from the exterior surface of said clamp to the exterior surface of the conductor it is around and a winding of semi-conductive stretch paper covering the exterior surfaces of said clamp and each said filling.

'4. A high tension cablejoint interconnecting a pair of oil-filled cables each having a central conductor, said joint comprising a jointing clamp having an interior .bore receiving the ends of the conductors of said cables,

said clamp having an intermediate exterior surface of greater circumference than the circumference of said conductors spaced from the surfaces of said conductors and extending substantially parallel to the surfaces of saidconductors and having end surfaces extending at an angle to'and between said exterior surface of said clamp and said surfaces of said conductors, plugs of insulation around said conductors and abutting said end surfaces at one end thereof, the exterior surface of each said plug, at said one end thereof, being substantially at the level of said exterior surface of said clamp and, p

at the other end thereof,'being substantially at the level of said conductorsurface, said exterior surface of said filling having a gradual taper intermediate said ends thereof, and a Winding of semi-conductive stretch paper covering the exterior surfaces of said clamp and said plugs. V 7

References Cited in the file of this patent UNITED STATES PATENTS 2,102,129 Rosch Dec. 14, 1937 2,322,702 Peterson" June 22,- 1943 2,386,185 Beaver et al. Oct. 9, 1945 2,834,828

Ebel May 13, 1958 

1. A HIGH TENSION CABLE STRUCTURE AT AN END OF A CABLE HAVING A CENTRAL CONDUCTOR, SAID STRUCTURE COMPRISING AT LEAST ONE CONDUCTIVE PART ELECTRICALLY CONNECTED TO AND ENCIRCLING SAID CONDUCTOR, SAID PART HAVING AN EXTERIOR SURFACE SPACED FROM SAID CONDUCTOR AND EXTENDING SUBSTANTIALLY PARALLEL TO THE SURFACE OF SAID CONDUCTOR AND HAVING AN END SURFACE EXTENDING AT AN ANGLE TO AND BETWEEN SAID EXTERIOR SURFACE OF SAID PART AND SAID CONDUCTOR SURFACE, A FILLING AROUND SAID CONDUCTOR AND ABUTTING SAID END SURFACE AT ONE END THEREOF, THE EXTERIOR SURFACE OF SAID FILLING, AT SAID ONE END THEREOF, BEING SUBSTANTIALLY AT THE LEVEL OF SAID EXTERIOR SURFACE OF SAID PART AND, AT THE OTHER END THEREOF, BEING SUBSTANTIALLY AT THE LEVEL OF SAID CONDUCTOR SURFACE, SAID EXTERIOR SURFACE OF SAID FILLING HAVING A GRADUAL TAPER INTERMEDIATE SAID ENDS THEREOF, AND A WINDING OF SEMI-CONDUCTIVE STRETCH PAPER AROUND SAID FILLING AND HAVING AN EXTERIOR SURFACE EXTENDING FROM SAID EXTERIOR SURFACE OF SAID PART TO THE SURFACE OF SAID CONDUCTOR. 